The invention relates to a method for forming electrically conductive circuits on a base board, and more particularly relates to a method for forming a resistor circuit on the base board, wherein a newly developed electrically conductive copper paste, which is specifically adapted to metal plating, is effectively used to form a two-layer circuits of first lower and second upper circuits on one side of the base board, and then a printable resistor paste is used to form a thinnest possible resistor circuit on the second upper circuits.
So far, it has been general to form a resistor circuit on a copper laminated print base board. In this case, a resistor with a lead or a chip-shaped resistor is soldered to a copper lamination circuit. The finished product is therefore bulky in addition to requiring so many processing steps and the resultant high cost including the cost of the resistor. Further according to such a conventional method, the loading density of the print base board is lower, and the reduction of product weight and of production processes is difficult. Moreover since the soldering operation is required, there have often been a misarrangement of leads and a misinsertion of resistors.
Further in the case of forming considerably complex circuits on the copper laminated print base board, it becomes necessary to electrically connect the circuits to each other. However according to the prior art; since it was impossible to form the circuits of more than two layers on one side of the print base board, such circuits were divided and etched on both sides of the base board and electrically connected to each other by the through-holes extended through the thickness of the base board.
Such prior art requires to attach the copper laminations on both sides of the base board, to etch the copper laminations along the designed circuits and to make holes extending through the base board by means of a specific device such as the NC device, and therefore the production cost is increased including the costs of materials and of processing steps, and moreover the production efficiency is low.
In order to improve the conventional method so as to form the electrically conductive circuits of more than two layers on one side of the base board more effectively at a lower cost, it has been required to use an electrically conductive copper paste which is excellent in the electric conductivity and adapted to a metal plating, especially to a copper plating and which is available at a lower cost. Generally the conventional electrically conductive copper paste is easily oxidized with a heat for hardening the paste, in contrast to the precious metal such as silver. The oxidization of copper powder in the paste will increase the electric resistance and decrease the soldering property. These defects have made the conventional electrically conductive paste practically useless. Further it has been required to activate the surface of the hardened electrically conductive copper paste by means of a catalyst so as to expose the copper powder from the resin paste so that the exposed copper powder may act as the binder, that is, so many neucleuses for the subsequent metal plating. Thus the conventional electrically conductive paste has required so many processing steps.
The Japanese utility model application Ser. No. 55-42460 discloses a specific method, in which a high dielectric resist of polybutadiene is used as a dilectric coat, an adhesive paste, for example, of 20% of phenol resin, 63% of copper powder and 17% of solvent is used to form designed circuits, the adhesive paste is thickened up to 20.mu. by means of a non-electrolytic plating, and then the plated adhesive paste is coated with copper so as to form the electrically conductive circuits of more than two layers on one side of a base board. The mentioned specific method has never been industrially reduced to practice.
The applicant has been engaged in studying new electrically conductive copper pastes for many years to eliminate the defects of the prior art as mentioned above and succeeded to provide such new electrically conductive copper pastes to be industrially employed. The newly developed electrically conductive copper pastes includes the electrically conductive copper paste ACP-020, ACP-030 and ACP-007P of Asahi Chemical Research Laboratory Co., Ltd. The electrically conductive copper paste ACP-020 is substantially composed of 80% by weight of copper powder and 20% by weight of synthetic resin, and is extremely excellent in the electric conductivity, but more or less deteriorated in the soldering property. The electrically conductive copper paste ACP-030 is substantially composed of 85% by weight of copper powder and 15% by weight of synthetic resin, and is slightly lower than the ACP-020 as to the electric conductivity, but excellent in the soldering property. In the last place, the electrically conductive copper paste ACP-007P is an improvement of the ACP-030 and may be subjected to a metal plating such as a copper chemical plating without using a catalyst. Namely the copper paste is extremely excellent in the metal plating property.